A uniform surface alignment of liquid crystals (LCs) is an important problem in practical applications of liquid crystal cells. In the case of uniform alignment, the direction of the average orientation of LC molecules on the substrate can be described by two angles—zenithal angle θ (the angle between the substrate and the direction of LC average orientation, also called the pretilt angle of the LC, and the azimuth angle φ (the angle in the plane of the substrate, measured between the director and some axis). In absence of external torques, the two angles have well-defined equilibrium values or range of values that are determined by the specifics of molecular interactions at the liquid crystal-substrate interface. These equilibrium values determine one, two, or more “easy axis” or “easy axes” directions. The angle θ can be used to classify the types of uniform alignment of LC. Three cases can be cited:
1) homeotropic (also known as perpendicular or normal) alignment characterized by preferential orientation of LC molecules in a direction normal to the film. In this case LC pretilt angle θ is equal to 90°.
2) planar alignment characterized by uniaxial ordering of LC molecules in plane of the aligning substrate (θ equals 0).
3) tilted alignment with the orientation axis obliquely oriented with respect to the aligning substrate. For this type of alignment 0°<θ<90°.
The first type of alignment implies that the azimuth angle φ is not specified, whereas 2) and 3) types are characterized by a well-defined value of the azimuth angle φ, when the medium adjacent to the LC is anisotropic. The azimuth direction might be degenerate, i.e., φ is not specified, when the adjacent medium is isotropic in the film plane.
As, a rule, homeotropic alignment of LC can be relatively easy obtained for both thermotropic and lyotropic LCs. Thermotropic materials acquire their mesomorphic (orientationally ordered) state when the material is within a certain temperature range. Lyotropic materials become mesomorphic when dissolved in some solvent (such as water), within an appropriate concentration range. The most common method of homeotropic alignment is a treatment of the aligning substrates with surfactant materials. In contrast, great skill is required to obtain planar or tilted alignment with desirable alignment parameters such as the azimuth angle φ, strength of anchoring, etc. The problem is challenging for both thermotropic and lyotropic LCs. The most common technique for such a controlled alignment is a unidirectional rubbing of special aligning films (e.g., polymer films) deposited at the bounding substrates. However, this method often hinders the further improvement of the devices based on LC cells because of several principal drawbacks. The rubbing process causes surface deterioration as well as generation of electrostatic charges and dust on the aligning surfaces. Besides, it is not convenient for the fabrication of LC cells having some special structure, for example, multidomain cells. The reason is that the rubbing method implies mechanical contact with aligning substrates.
To avoid the problem, a number of non-contact LC alignment methods has been suggested. Among them the photoalignment method is the most promising and intensively studied. Using this method, substrates are covered by photosensitive materials and subsequently irradiated with polarized UV or visible light. The photoalignment method allows controlling of LC anchoring and easy axis direction in both azimuthal and polar planes. This makes possible patterned alignment used to enhance viewing angles in nematic LCD. However, the photoaligning technique is usually accompanied with a low anchoring strength and relatively poor photo and thermal stability. Besides, LC alignment on the photoirradiated substrates is characterized by the pronounced image sticking effect which is a residual image when the controlling voltage is changed.
From the first sight, the main problem of photoalignment is a problem of useful materials. However, following literature data, practically all photoaligning materials developed up to this date more or less suffer from the drawbacks mentioned above. This gives the reasons to conclude that shortcomings of photoalignment are mainly associated with treatment procedure. As we believe, the action of UV/Vis light modifies the aligning surface only “softly” and so it is not capable to create strong boundary conditions for LC layers.
To overcome shortcomings of the conventional photoalignment method, M. Hazegava suggested to use deep UV irradiation. He showed that 257 nm irradiation causes LC alignment effect on the polymers, which are non-sensitive to conventional UVis light. One more radical solution is suggested by Chaudhari et al. It consists in oblique irradiation of the aligning polymer substrates with a collimated or partially collimated ion beam. This method provides excellent LC alignment on both organic and non-organic substrates. Later on, several modifications of the ion treatment method have been suggested. In the aligning substrate is bombarded with ions at normal incidence in the presence of an electric field, which is applied in the area close to the substrate. In this case the applied field is sufficient to redirect ions obliquely to the substrate. The other modification is proposed in where ion beam irradiation is used in combination with rubbing to produce two-domain patterning of the aligning substrate.
The advantages of deep UV irradiation and ion irradiation can be combined by the treatment of the aligning substrates with various kinds of plasma. The processing of LC substrates with the glow discharge was earlier applied for surface etching, grafting of the aligning surfaces with various atoms, as well as plasma polymerization. In comparison to prior art plasma methods that include deposition of various films [J. C. Dubois, M. Gazard, and A. Zann, 1974, Appl. Phys. Letters, 24(7), 29738-40; R. Watanabe, T. Nakano, T. Satoh, H. Hatoh, and Y. Ohki, 1987, Jpn. J. Appl. Phys., 26(3), 373, and A. I. Vangonen, and E. A. Konshina. 1997, Mol. Cryst. Liq. Cryst., 304, 507] and post-deposition treatments, mainly by bombardment with reactive ions [N. Shahidzadeh, A. Merdas, and W. Urbach, 1998, Langmuir, 14, 6594, 41-43; J. G. Fonseca, P. Charue, and Y. Galerne, 1999, Mol. Cryst. Liq. Cryst., 329, 597; and S. P. Kurchatkin, N. A. Muravyeva, A. L. Mamaev, V. P. Sevostyanov, and E. I. Smirnova, Patent of Russia No 2,055,384.], the technique of the present invention has a number of advantages. All the previously known methods listed above are reportedly capable of producing various values of zenithal anchoring coefficient and pretilt angle but not a uniform planar alignment; mostly because the substrates are placed in the gas discharge area where the plasma treatment is practically isotropic. Sprokel et al. [G. J. Sprokel and R. M. Gibson, 1977, J. Electrochem. Soc., 124(4), 559] proposed a directed plasma flux and anisotropic treatment, which resulted in a uniform planar alignment. This was achieved by the use of a modified r.f. plasma etcher in which reactive plasma was extracted and carried onto substrates by the gas stream. The technique of alignment set forth in this invention is extremely versatile, as it allows one to align both the thermotropic LCs such as the ones used in LC displays and lyotropic LCs, such as lyotropic chromonic LCs used in optical elements and biological sensors, see C. Woolverton et al., U.S. Pat. No. 6,171,802; O. D. Lavrentovich and T. Ishikawa, U.S. Pat. No. 6,411,354 and O. D. Lavrentovich and T. Ishikawa, U.S. Pat. No. 6,570,632. Moreover, all these liquid crystalline materials can be aligned at a broad variety of substrates, both inorganic and organic.